Programmed Cell Death. During embryonic development, normal cell turnover, and many other physiological and pathological processes, cells die by the activation of an internally programmed mechanism termed apoptosis or programmed cell death. This process appears to be under genetic and biochemical control, but little is known about the relevant genes and mechanisms. Low-dose ionizing irradiation of pregnant rats of 16-18-day gestation causes the rapid and massive death of proliferating and differentiating stem cells of the fetal forebrain. We are examining this system (a) to determine whether these cells die by apoptosis and (b) to study the dynamics of mRNAs coding for gene products potentially relevant to apoptosis. We found that gamma-radiation elicits within 3 hours nuclear pyknosis and fragmentation in undifferentiated cells in the cortical neuroepithelium and also elicits fragmentation of fetal brain DNA into an oligonucleosomal ladder pattern characteristic of apoptosis. This DNA fragmentation requires ongoing RNA and protein synthesis, as determined by the use of specific inhibitors. These characteristics suggest that radiation kills these cells by the mechanism of apoptosis. Northern blot analysis revealed that the abundances of most mRNAs tested (relative to total RNA), including those coding for p53, c-Myc, Bcl-2, and three housekeeping proteins, declined to 50-60% of the control levels by 3-5 hours after irradiation. In contrast, the mRNA for c-Fos, an early-response transcription factor, was increased to 20 times the control by irradiation. To identify novel induced genes induced by radiation, we prepared a subtractive cDNA library enriched in transcripts increased in irradiated fetal brain. Many clones were sequenced and found to represent transcripts only modestly increased in abundance (2-4- fold) or not decreased by irradiation. These results do not provide evidence for a requirement for dramatic induction of putative "cell death genes" in radiation-induced apoptosis. In another study, we also found that derangements of cellular protein phosphorylation by inhibitors of protein kinases (C and tyrosine-specific) or inhibitors of protein phosphatases elicit rapid apoptosis and oligosomal DNA fragmentation in P815 mouse mastocytoma cells.